The present invention relates to a process for manufacturing micromechanical devices containing a getter material and to the devices manufactured through this process. In particular, the invention relates to a process for manufacturing the devices comprising a step of joining together two wafers by melting at the interface therebetween, one of the wafers being made of silicon and the other one being made of a semiconductor, ceramic or oxidic material; the invention relates as well to the final devices and to particular getter materials capable of withstanding the process conditions.
Micromechanical devices are generally known as “MicroElectroMechanical Systems” or “MicroOptoElectroMechanical Systems”, and with their abbreviations MEMS and MOEMS (in the following reference will only be made to MEMS, by that also meaning MOEMS). These devices are formed of a sealed cavity, inside which micromechanical parts capable of carrying out preset movements or parts capable of interacting with electromagnetic radiation are present, in addition to auxiliary parts and electrical feedthroughs for both the power supply of the device and the transmission of the signal generated by the device to the outside. Examples of such devices are microaccelerometers, described in numerous patents such as U.S. Pat. Nos. 5,594,170; 5,656,778 and 5,952,572; miniaturized resonators, used in the field of telecommunications and particularly in the manufacture of mobile phones, described in U.S. Pat. Nos. 5,821,836 and 6,058,027; or IR miniaturized sensors, an example of which is described in U.S. Pat. No. 5,895,233.
At the end of the manufacturing process, various gases are contained in the cavity of a MEMS, being residual of the process or due to the degassing of the same cavity walls, which may interfere with the MEMS operation: for example, they can modify the thermal conduction in the system thus altering the temperature measure in the case of an IR sensor, that thus needs the best possible degree of vacuum in the cavity. Other MEMS devices do not have such a stringent requirement for extremely high vacuum levels. For instance, in accelerometers a low pressure of gas in the cavity helps to dampen the vibration of the moving part after it has been placed in motion. This allows fast recovery of the “rest” status of the device, making it more quickly ready for further movement detections. To this end, the manufacture of some MEMS foresees the backfilling of the cavity, prior to its sealing, with a given gas (e.g., a noble one) at pressures on the order of thousands of Pascal (Pa). In these cases too, however, it is necessary that the atmosphere in the cavity have a constant pressure and chemical composition, because variations in these parameters could alter the viscosity of the medium around movable parts, thus altering the measurements.
The achievement of very high vacuum degrees or of a constant atmosphere throughout the life of a MEMS device can be assured by introducing into the cavity a getter material, that is, a material capable of removing most non-noble gases. Getter materials usually employed are metals, such as zirconium, titanium or alloys thereof. Preferred is the alloy having the weight percent composition zirconium 80%-cobalt 15%-Rare Earths 5%, sold by SAES Getters S.p.A. under the trademark St 787. The use of getter materials in MEMS devices is described, for instance, in U.S. Pat. Nos. 5,952,572; 6,499,354; 6,590,850; 6,621,134; 6,635,509; and 6,923,625. MEMS are manufactured with technologies derived from the manufacturing of integrated semiconductor circuits, typically consisting of localized depositions of desired materials onto planar supports of glass, of quartz, of a ceramic material (e.g., silicon carbide) or of a semiconductor material (silicon is preferred), and selective removals of parts of layers of different material. In particular, the last generation MEMS, described in the following with reference to FIG. 1, are normally manufactured by welding two parts together, a first part 10 being commonly formed of a planar support, generally of silicon, on which the functional elements 11 and the auxiliary ones (these not shown in the drawing) are built, and a second part 12, which may be made of glass, quartz, ceramic or a semiconductor material, and generally has only the function of closing the device in order to protect the inner elements. As this second part is generally free from functional elements and thereby provides more free space, the getter material 13 is preferably arranged on this part, as described for instance in U.S. Pat. No. 6,897,551. The preferred technique for the deposition of getter material layers in this application is cathodic deposition, commonly known as “sputtering.” As known in this technique, a body, generally having a short cylinder shape (called “target”) made of the material intended to be deposited, and the support on which to form the deposit are arranged in a sealed chamber. The chamber is filled with a noble gas, generally argon, at a sub atmospheric pressure. By applying a potential difference of the magnitude of thousands of volts (or lower, depending on the configuration used) between the target (held at cathodic potential) and an anode, the noble gas is ionized and the ions so produced are accelerated towards the target, thus eroding it by impact. The eroded material deposits on the available surfaces, including the support. By employing masking systems provided with suitable openings, it is possible to restrict the area of the support where the deposit has to be formed. As an alternative, sputtering can be carried out under reactive conditions, namely, adding to the noble gas small percentages of a reactive gas, oxygen for instance, which reacts with the particles eroded by the target in gaseous phase, producing a deposit of the material that results from the reaction of the reactive gas with the particles. Once all the elements needed for the operation of the MEMS have been formed on the two parts, these are joined together by welding them along a line enclosing the elements of the device. The micro-device is thus sealed in a closed space 14 and is mechanically and chemically protected from the outer environment.
The welding can be accomplished by numerous techniques, known by the common definition of “bonding.” A first possibility consists in inserting between the two parts a malleable metal, such as indium, lead or gold, and causing this to melt and solidify, possibly while exerting pressure (“pressure bonding”). However, this technique results in not completely reliable weldings from the mechanical strength point of view. Another type of bonding is anodic bonding (mainly employed where one of the supports is made of glass or quartz and the other one is of silicon), wherein a potential difference of about 1000 V is applied between the two parts kept at a temperature of about 300-500° C. Under these conditions there is a migration of positive ions from the support kept at positive potential (e.g., sodium ions from the glass) toward the support kept at negative potential, and a migration of negative ions (e.g., oxygen from silicon) to the opposite direction. This material migration between the two supports results in the mutual welding thereof. Another possible technique is eutectic bonding, wherein a layer of metal or alloy, able to form a eutectic composition with the material of at least one of the two supports, is interposed between the two supports, so as to cause a localized melting in the welding area by a suitable thermal treatment. However, the eutectic bonding is not generally used when there are dangers of metallic contamination, e.g., when the eutectic bonding is carried out together with cMOS manufacturing processes. The getter materials presently available are those compatible with MEMS manufacturing processes, which use the above-described bonding techniques.
Another possible MEMS sealing technique is direct bonding, requiring the localized melting of the material of the supports. Achievement of a stable bonding through this technique generally requires high temperatures, e.g. about 1000° C. in the case of silicon. This technique can also use an intermediate layer, e.g. silicon oxide when bonding two silicon parts. Compared to the previously cited techniques, direct bonding allows a better bonding between the two parts to be welded, increasing both the adhesion force and the stability with respect to stress conditions, such as thermal and mechanical shocks, vibrations or thermal cycles. With such a type of bonding, the mechanical characteristics of the welding area are comparable with those of the material forming the welded parts. This type of bonding is used for devices which must have great reliability and durability (e.g., avionic applications).
However, it has been verified that getter materials used at present are not capable of withstanding the thermal treatments required by direct bonding. First, during these thermal treatments most getter materials undergo strong structural and morphological rearrangements, which may result in melting of the material deposit (e.g., in the case of the cited St 787 alloy). The minimum consequence observed is the nearly total loss of the gas sorption ability by the getter, while in the case of melting the getter material can “wet” the functional parts of the device and, after solidification, it can completely alter or prevent the operation thereof. In some cases, a partial evaporation and new condensation of the getter material on the adjacent surfaces has also been observed, with negative consequences on operation of the device. Another problem that has been observed with all the getter materials is that, if the deposit is formed on silicon, it detaches from the support during the cooling that follows the bonding operation, so that solid fragments can come into contact with the functional parts, thus jeopardizing efficiency of the device.